The present invention relates generally to the field of conductor insulation and, more particularly, to a method for detecting and measuring voids in conductor insulation systems of a size considerably smaller than would generally be considered an obvious manufacturing defect.
Voids and gaseous cavities originate in conductor insulation systems through a variety of mechanisms, including normal as well as improper manufacturing processes, severe or cumulative mechanical and environmental stresses, and differential thermal expansions.
As explained in more detail below, when a conductor insulation system contains voids or cavities of a sufficient size and density, electrical discharges occur therein. These discharges result in an increase in current flow through the insulation between the conductor and ground or between two adjacent conductors, and a consequent reduction in the amount of current which is able to be transmitted through the conductor(s).
Further, the presence of voids or cavities of a sufficiently large size within a conductor insulation system (when subjected to an electric field or potential of sufficient magnitude) facilitates the initiation and growth of "electrical trees," which also detrimentally affect the insulation system's conductivity and projected life.
An electrical tree consists of a number of tiny hollow channels that extend and propagate from voids or impurities present in a conductor insulation system. The hollow channels can contain or allow considerable unstable discharges which, in time, may initiate further tree growth until cable failure occurs.
As explained above, when voids are present in a conductor insulation system, the insulating walls in effect "erode" in time and cause dielectric breakdown.
Three processes cause this dielectric breakdown: (1) bombardment of the void's walls by ions and electrons produced when gases within the void become ionized (i.e., corona breakdown); (2) heat generated by the corona breakdown process; and (3) chemical reactions within the void, due to the formation of ozone.
Conventionally, manufacturers have been responsible for testing conductor insulation systems, such as cables, for obvious voids (i.e., manufacturing defects of a magnitude sufficient to allow immediate or premature cable insulation failure) before shipping the cables from the factory. The manufacturers' favored production test (i.e., the partial discharge test) is performed to cable specifications prepared by the Association of Edison Illuminating Companies (AEIC). The AEIC has only promulgated specifications for medium and high-voltage cable; currently, there are no specifications for low-voltage (i.e., 600 to 2000 Volts) cable, which is the most commonly used cable today.
The partial discharge test measures the discharge magnitude (Q), which is measured in pico-coulombs. Manufacturers are required to maintain the discharge magnitude below a specific level (i.e., not greater than 5 pico-coulombs) before shipping the cable.
The industry has accepted the partial discharge test because it uses the simplest measurement that can be made to date, and it can detect insulation degradation. However, because of the many cable failures that occur due to voids being present in cable, often after installation and over time, it is evident that the partial discharge test and other existing testing methods are not able to locate voids that can cause subsequent failure.
As can be appreciated, the goal of current testing methods, including the partial discharge test, is to ascertain whether partial discharges are present in a conductor insulation system before it is shipped to customers. Also, there are currently no nondestructive tests available to detect voids or other defects, or to estimate system condition, in installed conductor insulation systems. Consequently, the conventional approach to evaluating cable condition is by a "post-mortem" analysis after the cable has failed.
Because the presence of voids can ultimately lead to conductor insulation system failure, those industries, such as the nuclear power industry, where the integrity of power and control systems implemented by conductor insulation systems is critical, have long desired a method for assessing insulation integrity degradation (which is related to void size and density), and for identifying potential problems related thereto, in new and installed conductor insulation systems before partial discharge occurs.